Components for gas turbine engines manufactured from nickel and cobalt-based superalloys often contain defects from manufacturing or service. Because these components are costly, there is considerable incentive to repair them. However, due to the lack of suitable repair procedures, many parts are currently scrapped. The use of fabricated superalloy components is similarly limited by the absence of suitable joining techniques.
Several repair techniques have been previously developed and are currently being used in specific applications. Fusion welding of superalloys is difficult due to their tendency to form weld-related cracks. Thamburaj, et al., Int. Met. Rev., Vol. 28, No. 1, pp. 1-22 (1983). However, welding is possible using lower strength filler materials. Unfortunately, this limits the use of welding to low stressed areas.
Brazing is also commonly used to join or repair superalloy components. In several processes, the defects are cleaned using fluoride ions (U.S. Pat. No. 4,098,450) or hydrogen (J. C. Bake, Welding J., Res. Supp., Aug., pp. 559-566 (1971)) to remove oxides from the crack or flaw surfaces. A braze alloy is then flowed into the gap to effect the repair or form a joint. One problem with this technique is the adequacy of the cleaning procedures. If the crack surfaces are inadequately cleaned, incomplete joints will form. The second problem is that, as with weld joints, brazed joints are typically much weaker than the base alloy. Diffusion brazing is a variation of this procedure (U.S. Pat. No. 3,678,570) in which the filler alloy has a composition similar to the base alloy and strengths approaching the base metal levels can be obtained. However, to permit reasonable joining times to be obtained, very thin joint gaps are used to obtain these strength levels. Such thin gaps are not common in repair situations.
An alternative approach for repair and joining using braze filler materials employs wide gap brazing mixtures which are mixtures of a higher melting alloy and a brazing alloy which may be of the conventional type (U.S. Pat. No. 3,155,491) or of the diffusion brazing type (U.S. Pat. Nos. 4,381,944 and 4,008,844). The higher melting alloy is typically of a composition similar to the base metal being joined, while the brazing alloy usually contains 2-4% of boron and silicon. Mixtures containing from 30-70% braze alloy have been used. An inhomogeneous structure of alloy particles in a braze alloy matrix is typically obtained. Diffusion heat treatments can be used to make the microstructure more uniform. However, the mean level of melting point depressants in the joint is still 1-3% of boron and silicon. For this type of repair, the defect is ground out prior to the use of the braze mixture, thus eliminating the problem of inadequate cleaning. However, the strength of the joint is still substantially lower than that of the base metal, limiting such repairs to low stressed areas. The use of this technique is also limited by the compatibility of the filler with the protective coatings used on turbine components. Antony & Goward, Superalloys, pp. 745-754 (1988). The high level of melting point depressants such as silicon and boron interfere with the performance of the coatings.
In one description of wide gap brazing, it is reported that the high melting point alloy can be sintered into the joint to form a porous structure which is infiltrated with braze alloy in a subsequent step. Chasteen & Metzger, Welding J. Res. Sup., pp. 111s-117s (Apr. 1979). This results in a distinctly nonuniform structure of powder particles in a matrix of braze alloy which has mechanical properties lower than the base metal and higher than a pure braze joint.
It has been suggested that liquid phase bonding can be used to join nickel base superalloys. M. Jeandin, et al., High Temp. Tech., Vol. 6, No. 1, pp. 3-8 (February 1988). In this process, conventional superalloy powders are placed in the joint, heated to a temperature where they are partially liquid and accelerated sintering occurs. Unfortunately, at these temperatures the base alloys being joined are also partially liquid making the technique unsuitable for repair or joining of finished shape parts.
The above references are hereby incorporated by reference.